This invention relates to a method of reducing ketones and aldehydes to the corresponding alcohol or alkyl group, using H2 gas as the stoichiometric reductant, and organometallic ruthenium complexes as the catalysts.
The homogeneous catalyzed reduction of ketones and aldehydes to various products is an important synthetic reaction in industry. Ruthenium complexes are well known catalysts for the reduction of ketones and aldehydes to various end products. Often phosphorus-containing ligands are used to complex the ruthenium (U.S. Pat. No. 5,614,641, U.S. Pat. No. 4,418,227, and Ohkuma, et al., J. Amer. Chem. Soc., 1998, 120, 1086). Typically, phosphorus-containing ligands are expensive, difficult to make and handle, and sensitive to oxygen.
Chinn, et al, (Organometallics 1989, 8, 1824-1826) described the synthesis and spectroscopic properties of the unstable dihydrogen complex [Cp*Ru(CO)2(H2)]+ which was found to decompose to {[Cp*Ru(CO)2]2(xcexc-H)}+OTfxe2x88x92, where Cp* indicates a xcex75-C5Me5 group. The dihydrogen complex is synthesized under mild conditions, has no need for oxygen sensitive ligands (e.g., triaryl phosphines, trialkyl phosphines) or nitrogen-based ligands, and is tolerant to acid and water. However, no mention is made of the catalytic activity of any of these complexes.
The invention is directed to a process for the reduction of a ketone or aldehyde, comprising contacting a compound of the formula R1xe2x80x94C(xe2x95x90O)xe2x80x94R2 with hydrogen in the presence of a catalytically effective amount of a catalyst precursor having the formula {[CpM(CO)2]2(xcexc-H)}+Qxe2x88x92, wherein M is selected from the group consisting of Ru and Fe, R1 is selected from the group consisting of hydrogen, substituted and unsubstituted alkyl and aryl groups, R2 is selected from the group consisting of substituted and unsubstituted alkyl and aryl groups; Cp is xcex75-C5R5 wherein R is selected from the group consisting of hydrogen, substituted and unsubstituted C1-C18 alkyl and aryl groups; and Qxe2x88x92 is a non-coordinating or weakly coordinating non-reactive anion. R1 and R2 may form a ring together.
Preferably, M is Ru, Qxe2x88x92 is OSO2CF3xe2x88x92, R is selected from the group consisting of hydrogen and methyl, and the compound of the formula R1xe2x80x94C(xe2x95x90O)xe2x80x94R2 is selected from the group consisting of cyclooctanone, levulinic acid, levulinic acid methyl ester, benzylacetone, 3-heptanone, 4-methoxy-acetoacetate, 3-pentanone and propanal.
Also preferably R1 is selected from the group consisting of hydrogen and substituted and unsubstituted alkyl groups, R2 is selected from the groups consisting of substituted and unsubstituted alkyl groups, and the reduction product is an alcohol.
In the case where R1 and R2 have not formed a ring, the process can further comprise the cyclizing of the alcohol to form the corresponding lactone.
The invention is further directed to a process to reduce a ketone or aldehyde to form a compound of the formula R1xe2x80x94CH2xe2x80x94R2, comprising contacting a compound of the formula R1xe2x80x94C(xe2x95x90O)xe2x80x94R2 with hydrogen in the presence of a catalytically effective amount of a complex having the formula {[CpRu(CO)2]2(xcexc-H)}+Qxe2x88x92, wherein R1 is selected from the group consisting of hydrogen, substituted and unsubstituted alkyl and aryl groups; R2 is selected from the group consisting of unsubstituted and substituted aryl groups, Cp is xcex75-C5R5 wherein R is selected from the group consisting of hydrogen, and unsubstituted and substituted C1-C18 alkyl and aryl groups; and Qxe2x88x92 is a non-coordinating or weakly coordinating non-reactive anion. R1 and R2 may form a ring together.
Preferably Qxe2x88x92 is OSO2CF3xe2x88x92 and R is selected from the group consisting of hydrogen and methyl, and the compound of formula R1xe2x80x94C(xe2x95x90O)xe2x80x94R2 is selected from the group consisting of 1-acetonaphthalene and acetophenone.
In the present description, xe2x80x9calkylxe2x80x9d means an alkyl group containing up to 18 carbon atoms. Common examples of such alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, s-butyl, isobutyl, pentyl, neopentyl, hexyl, heptyl, isoheptyl, 2-ethylhexyl, cyclohexyl and octyl. The alkyl group may be linear, branched, or cyclic.
By xe2x80x9csubstitutedxe2x80x9d is meant the addition of one or more substituent groups to a compound or functional group. Said substituent groups do not cause the compound to be unstable or unsuitable for use in an intended reaction, and are inert under reaction conditions. Substituent groups which are generally useful include nitrile, ether, ester, halo, amino (including primary, secondary and tertiary amino), hydroxy, oxo, vinylidene or substituted vinylidene, carboxyl, silyl or substituted silyl, nitro, sulfinyl, and thioether. Highly basic substituents are generally not suitable in the process of present invention unless previously protonated with acid or protected by a suitable protecting group.
By xe2x80x9carylxe2x80x9d is meant an aromatic carbocyclic group having a single ring (e.g., phenyl), multiple rings (e.g., biphenyl), or multiple condensed rings in which at least one is aromatic, (e.g., 1,2,3,4-tetrahydronaphthyl, naphthyl, anthryl, or phenanthryl), which may be mono-, di-, or trisubstituted with halogen, lower alkyl, lower alkoxy, lower alkylthio, trifluoromethyl, lower acyloxy, aryl, heteroaryl, or hydroxy groups. By xe2x80x9carylxe2x80x9d is also meant heteroaryl groups where heteroaryl is defined as 5-, 6-, or 7-membered aromatic ring systems having at least one heteroatom selected from the group consisting of nitrogen, oxygen and sulfur. Examples of heteroaryl groups are pyridyl, pyrimidinyl, pyrrolyl, pyrazolyl, pyrazinyl, pyridazinyl, oxazolyl, furanyl, quinolinyl, isoquinolinyl, thiazolyl, and thienyl. Said heteroaryl groups may contain substituent groups including halogen, lower alkyl, lower alkoxy, lower alkylthio, trifluoromethyl, lower acyloxy, aryl, heteroaryl, and hydroxy.
The following definitions are used herein:
Arxe2x80x2=3,5-bis(trifluoromethyl)phenyl
Cp*=xcex75-C5Me5 
HOTf=CF3SO3H
OTfxe2x88x92=OSO2CF3xe2x88x92
OTf=OSO2CF3 
The present invention provides a process to prepare an alcohol via the hydrogenation of a ketone or aldehyde by contacting a compound of the formula R1xe2x80x94C(xe2x95x90O)xe2x80x94R2 with hydrogen in the presence of a catalytically effective amount of a catalyst precursor, to form an alcohol, wherein R1 is selected from the group consisting of hydrogen, substituted and unsubstituted alkyl and aryl groups; R2 is selected from the group consisting of substituted and unsubstituted alkyl and aryl groups; and R1 and R2 may form a ring together. The catalyst precursor is of the formula {[CpM(CO)2]2(xcexc-H)}+Qxe2x88x92 where Qxe2x88x92 is a non-coordinating or weakly coordinating non-reactive anion, M is selected from the group consisting of Fe and Ru, and Cp is xcex75-C5R5 wherein R is selected from the group consisting of hydrogen and substituted and unsubstituted C1-C18 alkyl and aryl groups. Preferably Qxe2x88x92 is OSO2CF3xe2x88x92, R is hydrogen or methyl and M is Ru.
Preferred compounds of formula R1xe2x80x94C(xe2x95x90O)xe2x80x94R2 include cyclooctanone, levulinic acid, levuliniic acid methyl ester, benzylacetone, 3-heptanone, 4-methoxyacetoacetate, 3-pentanone, and propanal.
By xe2x80x9calcoholxe2x80x9d is meant a compound containing an alcohol functionality. By xe2x80x9cketonexe2x80x9d it is meant a compound containing a ketone functionality. By xe2x80x9caldehydexe2x80x9d it is meant a compound containing an aldehyde functionality.
The invention also provides a process to cyclize the alcohol to form a corresponding lactone. This may occur spontaneously under the reaction conditions when the keto group of an ester or carboxylic acid is in the xcex3, xcex4 or higher position relative to the carbonyl group of the reacting aldehyde or ketone, forming a 5-, 6- or higher membered ring, and eliminating an alcohol in the case of an ester, and water in the case of a carboxylic acid. Two examples of this process are illustrated below. The preferred compound for this embodiment is levulinic acid. 
The invention also provides a process to reduce a ketone or aldehyde to form a compound of the formula R1xe2x80x94CH2xe2x80x94R2, comprising contacting a compound of the formula R1xe2x80x94C(xe2x95x90O)xe2x80x94R2 with hydrogen in the presence of a catalytically effective amount of a catalyst precursor, wherein R1 is selected from the group consisting of hydrogen, substituted and unsubstituted alkyl and aryl groups, R2 is selected from the group consisting of unsubstituted and substituted aryl groups, and R1 and R2 may form a ring together. The catalyst precursor is of the formula {[CpM(CO)2]2(xcexc-H)}+Qxe2x88x92 where Qxe2x88x92 is a non-coordinating or weakly coordinating non-reactive anion, M is selected from the group consisting of Fe and Ru, and Cp is xcex75-C5R5 wherein R is selected from the group consisting of hydrogen and substituted and unsubstituted C1-C18 alkyl and aryl groups. Preferably R is hydrogen or methyl, Qxe2x88x92 is OSO2CF3xe2x88x92 and M is Ru. Preferred compounds of the formula R1xe2x80x94C(xe2x95x90O)xe2x80x94R2 include 1-acetonaphthalene and acetophenone.
When either R1 or R2 is a substituted or unsubstituted aryl group, the reaction will follow the path below under most reaction conditions: 
In some instances when an aryl substituent is present, only the alcohol or a mixture of the alcohol and alkyl product can be obtained. When neither of the substituents contains a hydrogen atom on the carbon atom that is alpha to the oxo group only the alcohol is obtained.
The catalytically active species for both processes is believed to be [CpM(CO)2]H in combination with one or both of the transient complexes [CpM(CO)2(xcex72-H2)]+ and [CpM(CO)2]+, all of which are generated under the reaction conditions. Any synthetic route may be used that leads to the same reactive species, such as use of CpM(CO)2OQ, [CpM(CO)2]2 and H+Qxe2x88x92, or CpM(CO)2H and H+Qxe2x88x92 as catalyst precursors.
The catalyst precursor, {[Cp*Ru(CO)2]2(xcexc-H)}+OTfxe2x88x92, was prepared in the following manner. In a drybox, a flask was charged with Cp*Ru(CO)2H (2.05 g, 6.986 mmol, prepared as described in Fagan et al., Organomet., 1990, 9, 1843). The Cp*Ru(CO)2H was dissolved in 2 ml of CH2Cl2. To this flask was added triflic acid (310 xcexcl, 3.5 mmol) slowly, resulting in vigorous gas evolution (H2). Diethyl ether (10 ml). was then added to the flask and a yellow solid precipitated out of solution. The yellow solid was isolated by filtration and rinsed twice with a minimum amount of diethyl ether.
Weakly coordinating anions are known to those skilled in the art. Such anions are often bulky anions, particularly those that may delocalize their negative charge. The coordinating capability of such anions has been discussed in the literature; see, for instance, W. Beck, et al., Chem. Rev., vol. 88, p. 1405-1421 (1988), and S. H. Strauss, Chem. Rev., Vol. 93, p. 927-942 (1993). Weakly coordinating anions suitable for the processes of the present invention include OSO2CF3xe2x88x92(herein abbreviated as OTfxe2x88x92), SO42xe2x88x92, HSO4xe2x88x92, BF4xe2x88x92, PF6xe2x88x92, SbF6xe2x88x92, BPh4xe2x88x92, and BArxe2x80x24xe2x88x92 where Arxe2x80x2=3,5-bis(trifluoromethyl)phenyl. Most preferred is OTfxe2x88x92.
The processes of the present invention are carried out under an atmosphere of hydrogen gas or any mixture of hydrogen gas with other gases that do not interfere with the desired reaction, such as nitrogen, helium, neon and argon gases. Other in situ sources of hydrogen can also be used. The partial pressure of the hydrogen should be between about 14 to about 1000 psi (0.1 to 7.0 MPa), preferably about 14 to about 800 psi (0.1 to 5.5 MPa). The temperature range is about 40xc2x0 C. to about 120xc2x0 C., preferably about 65xc2x0 C. to about 110xc2x0 C. The temperature and pressure range can vary depending on the selection of solvent, providing the aldehyde or ketone and solvent remain in the liquid phase.
The processes may be run neat or a suitable solvent may be used. Suitable solvents are non-coordinating, non-basic, inert and able to partially dissolve the catalyst precursor under the reaction conditions. The solvents may be deuterated for ease of analysis. Preferred solvents are dichlorobenzene, sulfolane (tetramethylene sulfone; tetrahydrothiophene-1,1-dioxide) and CH2Cl2.